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                                                                             Battery monitoring
                                                                                                                                        by Loic Moreau, LEM

The infrastructure that underpins contemporary society must operate with very high reliability. In contrast to the advanced technology deployed
in data storage centres, internet server farms and communications central offices, most of them rely for near 100% “up-time”, or system-
availability, on a very mature technology; the lead-acid battery.



It is standard practice to provide such          voltage. Establishing the battery condition             combination of analogue and digital
critical nodes, and many other essential         by measuring electrolyte specific gravity               techniques implemented in a single,
services, with back-up power, and the first      is not possible with the sealed VRLA                    custom-designed, SoC (system on chip)
layer of that back-up is usually an inverter     construction. Conventionally, the only way              integrated circuit. This device, deployed in
that draws on banks of valve-regulated           to verify capacity has been to discharge                a module that measures terminal voltage,
lead-acid (VRLA) cells.                          the complete batter y under controlled                  internal cell temperature and internal
                                                 conditions, requiring it to be taken out of             impedance, is a key element in the design
This technology continues in widespread
                                                 service.                                                of a system that can deliver accurate
use for a variety of reasons, not least
                                                                                                         measurements within a budget that is
the fact that the cells are economical,          Deep discharge also reduces the life
                                                                                                         affordable for the majority of standby
and offer excellent reliability. Excellent:      of lead-acid cells; in an installation that
                                                                                                         system configurations.
but not perfect. VRLA cells do have a            regularly discharge-tests its batteries, and
finite life (a design life of 12 years is        in which the primar y power supply has                  C e l l t e m p e r a t u r e s, a n d / o r i n t e r n a l
common), and in normal use in critical-          high reliability, the test regime will be the           resistance values, that increase in an
system standby power they are routinely          dominant factor in determining battery                  exponential manner, (Fig. 1) is indicative
replaced on timed basis. Failures can,           service life.                                           of impending failure, and a data logging
and do, still occur. In a typical standby        Newer, non-intrusive electronic methods                 system that monitors trends in the data
supply, the cells do exactly as the name         of continuous monitoring can detect                     over time will, potentially, identify an
implies – they are held at full charge           impending failures in individual cells,                 impending failure. Each Sentinel III module
awaiting failure of the primar y supply.         saving cost while maintaining full system               has an external temperature measurement
This is achieved by maintaining a small,         availability. Early versions of such systems            probe or patch that is applied directly to
continuous “ float ” charge. If the float        typically measured parameters such as                   the case of the cell or battery, to give the
charge current is below a certain limit,         cell or monobloc (the battery-industry term             most accurate possible tracking of cell
the gases produce by electrolysis within         for multiple cells in a single case) voltage            temperature.
the cell will recombine. In this condition,      – despite the known limitations – plus
                                                                                                         An established technique makes an
cells can be vulnerable to damage if the         charge/discharge current and ambient
                                                                                                         assessment of internal resistance while
float-charge voltage is even a little higher     temperature. Some systems attempt, with
                                                                                                         the cell is in service and on-charge. Exact
than the standard value of 2,27 V per            varying degrees of success, to measure or
                                                                                                         implementations var y, but typically, a
cell. A small overvoltage will cause the         infer the cells’ internal resistances.
                                                                                                         small AC voltage is superimposed on the
electrolyte to produce more gas than the         LEM ’s Sentinel system pioneered the                    DC of the float charge; measuring the
recombination chemistry can cope with,           transition from reliance on simple analogue             AC voltage and current yields the internal
and this will escape through the valve.          measurements of basic parameters, and                   resistance. The method has limitations,
Excessive cell temperatures, even with           is now in its third generation. It uses a               partly to do with the very shape of the
correct charging voltage, will also lead
to loss of electrolyte.

Other failure modes include early sulphating,
poor connection between the posts and
the grids, poor connection between
the grids and the plates, stratification of
electrolyte and accelerated grid corrosion.
An infrequent, but catastrophic, mode
of failure is thermal runaway, which is
peculiar to VRLA batteries and can result
in fire or explosion. The only way to detect
its onset is to monitor the internal cell
temperature.

It is recognised that simply monitoring
the cell voltage gives ver y little
information about declining capacity of a
lead-acid cell. A failing cell will frequently
present nominal voltage until it is called
on to deliver significant current, when it
                                                         Fig. 1: Cell internal impedance is not necessarily a good indicator of impending failure.
will exhibit severely reduced capacity               The exponential shape of the curve means that early-stage failure is hard to detect, but the later
with premature collapse of its terminal                                              decline in performance is rapid.

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APPLICATION
                                                                                                                      that can be removed during the testing
                                                                                                                      process.

                                                                                                                      Fi g. 4 s h o w s t h e p r o g r e s s i o n o f t h e
                                                                                                                      parameters over the life of a cell. The same
                                                                                                                      characteristics are also demonstrated
                                                                                                                      during a discharge, or loss of capacity. All
                                                                                                                      of the resistance aspects of the equivalent
                                                                                                                      circuit follow a similar curve; they do not
                                                                                                                      change greatly during the early stages of
                                                                                                                      failure, or of capacity loss. If resistance is
                                                                                                                      used as the main indicator of the state
                                                                                                                      of health of a cell, it will not give any
                                                                                                                      significant indication until the capacity loss
                                                                                                                      is greater than 25 – 30%. Since the industry
                                                                                                                      standard is to replace batteries that fall
                                                                                                                      below 80% of specified performance,
                                                                                                                      it is clear that possible failure must be
                                                                                                                      identified far earlier.
                      Fig. 2: The Randles equivalent circuit for an electrochemical cell.
                                                                                                                      There is, however, one parameter in the
                                                                                                                      Randles equivalent circuit that does
                                                                                                                      change in the early stages of cell failure
                                                                                                                      (except purely metallic corrosion, which
                                                                                                                      will be revealed by an increase in the
                                                                                                                      parameter Rm), and that is Cdl, the
                                                                                                                      double layer capacitance. Its behaviour
                                                                                                                      is indicated in the lower curve in Fig. 3;
                                                                                                                      again, the shape of the curve is similar for
                                                                                                                      a healthy cell during normal discharge,
                                                                                                                      or a failing cell that is supposedly fully
                                                                                                                      charged.

                                                                                                                      The monitoring technique
                                                                                                                      A detailed description of the technique is
                                                                                                                      beyond the scope of this article, but it can
                                                                                                                      be summarised as follows.

                                                                                                                      The test signal is applied on a cell-by-
                                                                                                                      cell basis, with no requirement to drive
                                                                                                                      high currents through complete blocs
                                       Fig. 3: The battery monitor module.
                                                                                                                      or batteries, and no disturbance of DC
                                                                                                                      connections to external systems. The
                                                                                                                      original algorithms were developed using
exponential curve. A cell that is about to                 the metallic resistance and represents the                 a bipolar test signal, but unipolar signals
fail can be well along the path to failure                 resistance of the metal and the jointing                   proved to be more reliable. However,
before the data logger identifies the trend:               between components.                                        unipolar signals experienced DC drift
conversely, by the time the problem is                                                                                during the test. Simply removing this trend
                                                           Re is the electrolyte resistance: electrolyte
spotted, the cell may only be a short time                                                                            does not preserve the characteristics of the
                                                           loss can be one of the main causes of
from complete failure.                                                                                                data set necessary for correct parametric
                                                           premature failure. Cdl is the double layer
L E M, i n a s s o c i a t i o n w i t h a c a d e m i c   capacitance, a function of the effective                   estimation. By rearranging the var ying
partners, has i mpl emen ted a mu ch                       plate area and the dielectric strength of                  frequency signal pulses that comprise the
more sophisticated algorithm that                          the electrolyte.                                           test signal, in a frequency sweep, the cell
detects deteriorating cell performance                                                                                voltage response can be made to follow
at a much earlier stage. The result is an                  Rct is the transfer (Faradic) resistance                   a predetermined curve.
extremely dependable test method,                          which arises due to limitations in the
                                                                                                                      Once the underlying drift curve becomes
which penetrates well into the cell ’s                     rates of chemical reaction kinetics at
                                                                                                                      uniform, firmware algorithms can be
energy layer, to ensure the highest possible               the plate/electrolyte interface. Wi, the
                                                                                                                      designed to model the drift and remove
degree of reliability. It is based on the so-              Warburg impedance, is representative of
                                                                                                                      it, resulting in a mean zero voltage data
called Randles equivalent circuit, which                   a diffusion mass transport process. It is a
                                                                                                                      set suitable for direct input to the Sentinel
represents an electrochemical cell as a                    low frequency component, not present
                                                                                                                      algorithms. This method can reduce drift
network of electrical elements, each of                    during discharge.
                                                                                                                      errors to less than 0,1% without causing
which is related to a physical aspect of
                                                           After separating out these equivalent                      significant distortion of the data set.
the cell’s construction.
                                                           c i r c u i t c o m p o n e n t s, e a c h o f w h i c h   Algorithms can then be applied to the
Each element of the Randles circuit (Fig. 2)               represents some per formance-limiting                      measured waveforms, and the equivalent
models a physical process and/or a failure                 factor, the cell’s energy source is shown                  circuit parameters can then be derived
mode of an electrochemical cell. Rm is                     as a pure electrical generation element                    with a high degree of accuracy.

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                                                                                                                                  APPLICATION
Much of the measurement functionality and
algorithmic processing has been integrated
into a single integrated circuit (system-on-
chip, or SoC). Sentinel modules can measure
at an individual cell level, or at monobloc
level (2 to 12 V). Up to 250 measurement
points, implemented in modular form, can
report via a proprietary data bus to a battery
data logger, the S-box. In a large battery
installation, several such streams of data
can be combined and made available
to a variety of local or remote upstream
supervisory systems via standard buses or
an Internet connection, using a web server
that is integrated in the S-box.

Having had the measurement SoC
determine ever y cell ’s real condition,                              Fig. 4: Randles parameter progression over the cell lifetime or discharge.
detecting impending failures is not                                  The various resistance parameters show the same shape of curve while the
the only benefit that a sophisticated                               double-layer capacitance, in contrast, exhibits an early detectable change.
monitoring architecture can provide;
several other features and services can
                                                   cell failures is essential, but prematurely                     drift and virtually no reminisce after a high
be programmed.
                                                   replacing cells that are not approaching                        current discharge that will further improve
For example, the cells of a battery often          end-of-life is expensive.                                       measurement fidelity. Advances such as
have differing internal impedances. Over                                                                           these can change the role of the battery
time, this can cause problems. The SoC             In addition to voltage, impedance and                           monitor from being an expensive addition,
intelligent control system allows such cells       discharge per formance per cell, LEM                            to that of an extremely cost-effective
to be quickly detected, and a system,              monitors internal cell temperature as                           integral life management system.
known as terminal voltage optimisation,            standard; a world-first. It is also developing
can divert the float current around the cell                                                                       Contact Mervyn Stocks,
                                                   a f l u x g a t e -t e c h n o l o g y f l o a t- c h a r g e   Denver Technical Products,
that cannot tolerate further charging.
                                                   transducer capable of better than 10 mA                         Tel 011 626-2023,
Active charge management for extended              resolution, with little or no temperature                       mstocks@mweb.co.za v
life:
The float current in VRLA cells is higher than
that of flooded types, for the same terminal
voltage. This can accelerate positive plate
corrosion and reduce the useful life of the
cell by up to 30%. Removing the float
charge for a proportion of service life can
reduce this effect. A useful side effect of life
cycling is its ability to reduce the possibility
of thermal runaway. A cell-mounted
module can also provide a lifetime log of
terminal voltage and temperature to help
manufacturers and users.

Exhaustive discharge protection:

It is normal in charger/UPS systems, and
even batter y monitors, to preserve the
battery by terminating a discharge based
on the average cell voltage. However,
weaker cells can be of a much lower
terminal voltage than the battery average
and may be exhaustively discharged
well before the battery has reached its
termination voltage. This can damage
these cells permanently. A highly accurate,
dynamic "Time to run" algorithm has been
developed to give warning if any individual
cell is approaching exhaustion.
Monitoring of standby battery parameters
must be as comprehensive as possible,
to produce results that most accurately
represent the state of the batter y. This
is not only a technological issue, but
an economic one. Avoiding in-service


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